Caesar and the whole Egypt campaign could be argued to stem for a need to assure grain exports to Rome but since it was not the stated reason, it does not really fit as an answer. Britain had a lot of tin as well so that could have been a major reason to invade.
–
SardathrionFeb 27 '12 at 7:24

1

@Sardathrion - I would consider an answer that conclusively proves that there were no other equally important reasons. I'm guessing that tin wasn't the main reason for Britain conquest
–
DVKFeb 27 '12 at 10:57

1

@Sardathrion - was that to assure Rome's access to timber, or specifically to prevent Carthage's access to limit its naval power/sea trade? May be I should make that into a separate question :)
–
DVKFeb 27 '12 at 11:14

5 Answers
5

Whele senators and ordinary civilians could justify a war with such considerations, the official pretexts for the wars were always different. All wars Rome conducted were officially motivated by international law. Particular motivations being:

Defending the allies (first and second Punic wars, Gallic war)

Breach of a treaty by the other party (second Punic war)

Invitation by a foreign pretender to the throne to help him against his adversary (invasion of Judea)

Pacification of warlike tribes (Caesar's invasions of Britain and Germany)

and so on.

It should be noted that "we just will conquer you" statements are quite rare in world's history. This is because if you use such arguments, you will experience problems in the future with concluding any treaties with other peoples, thus diplomatically unwise.

The most close possibly being Islamic conquests which were based on theological arguments.

While I agree with the premise that publicaly there is always some sort of plausible (or implausible) excuse, I mean there may be internal, private letters or speaches stating real reasons.
–
DVKFeb 28 '12 at 9:42

Even the Islamic conquest just as Crusades are based largely on imperialistic/economical motivations. We should distinguish real motifs from official recorded propaganda.
–
GregOct 29 '14 at 1:59

The only (to my knowledge) province that Rome brought under its control primarily based on a resource need was Egypt.
Egypt, and to a lesser extent, north Africa, were the so-called "granary of Rome". Egypt was a necessary supplier of grains in a time where Rome (the city) and the standing army were growing, and an increasing number of citizens was relying on handouts by politicians or the state - the so-called "plebs frumentaria". At its apex Rome imported approximately 350000 tonnes of grains each year and 200-300 thousand citizens were living off handouts.
Egypt also offered good connections to Asian trade routes. But if trade is considered a resource, then the Punic wars and the Hellenic wars might also be considered a resource driven assessment.

As user357320 already mentioned, the annexation of Dacia and Iberia also had some economic agendas, but that was not the main reason given at the time.

A bit of cart before the horse here. While Africa and Egyptian grain was a staple for the later Empire, the takeover of both was based on other, political motives entirely.
–
OldcatOct 29 '14 at 0:33

@Oldcat yes there was the whole Octavian vs. Mark Anthony business, and Cleopatra supporting him militarily. That was the reason why they marched, or rather, shipped troops there. However the reason for annexing it was its value. You might say they conquered it on a "well, since we're already here, so lets just conquer it"-logic, but that's not how roman conquests worked. There are plenty of punitive expeditions that didn't end in conquest but just did what they started out to do: punish some action or neutralize some threat. Conquest were motivated by strategical decisions...
–
MatthaeusOct 29 '14 at 0:42

That value was primarily just cash, not grain. It was too rich to allow anyone to take. That's why senators were not even allowed to visit Egypt by law in the Empire without permission. Still not an economic motive until later.
–
OldcatOct 29 '14 at 0:44

...be it to expand the territory to a natural border, or to obtain some advantage (tributes, access to trade routes, keeping regional powers down, access to resources, access to land for veterans). Especially during late imperial rule, they were very much aware of the difficulties to support further expansion, and weighed usefulness against costs of garrisoning and administering the territory.
–
MatthaeusOct 29 '14 at 0:46

At one point, the Romans extended the frontier in central Germania to swallow up a modest bit of land that was rich in silver mines. IIRC this was around the time of the Emperor Domitian.

Sources: The primary source was from The Grand Strategy of the Roman Empire by Edward Luttwak. It discusses the annexation and mentions the region as securing some fertile land and supporting a friendly tribe, as well as pressing on the hostile Chatti. It also provided a buffer for Maintz and the flank of the annexation of the Agri Decumantes to the south.

However I did find a supporting remark from Tacitus' Annals Book XI, 20, from the time of Claudius:

Nor was it long before the same distinction was gained by Curtius Rufus, who had opened a mine, in search of silver-lodes, in the district of Mattium. The profits were slender and short-lived, but the legions lost heavily in the work of digging out water-courses and constructing underground workings which would have been difficult enough in the open. Worn out by the strain — and also because similar hardships were being endured in a number of provinces — the men drew up a private letter in the name of the armies, begging the emperor, when he thought of entrusting an army to a general, to assign him triumphal honours in advance.

Here Curtius Rufus gets triumphal ornaments by sending his troops over the border to do some quick work Silver Mining in Germany. This is the same area that the lines would later extend out to encompass a generation or so later.